Sir: "Why should we worry about 'mad cow' disease when the incidence among vicars is higher than among farmers?" Peter Popham reports that there is a statistical excess of cases of Creutzfeldt-Jakob Disease (CJD) in cattle farmers compared with the general population, but seeks to dismiss this because at least one other occupational group, vicars, has a higher rate of reported incidence of CJD (11.8 cases per million vicars, against fewer than two cases per million cattle farmers).

The higher rate among vicars is irrelevant. There are scores of occupational groups commonly distinguished in epidemiological work, and the rate of incidence of any disease inevitably varies across those groups, even if there is in fact no link between the disease and occupation.

So, even when there is no such link, there is inevitably some occupational group that has the highest rate of incidence; and the rate of incidence in that group is typically very high, only because it has to be higher than all the others. Thus a rate as high as 11.8 cases per million in the highest group is not unexpected.

Moreover, it might be noted that a calculated rate of 11.8 cases per million would arise from a single reported case in an occupational group with around 85,000 members: and how many vicars are there? The clergy has no special cause for concern.

So why should we worry? Because if one considers a priori what findings might represent evidence of the infection of humans by BSE ("mad cow disease"), an excess of CJD cases in cattle farmers would surely qualify (as would an excess of cases in the meat-processing industry, for example). And such an excess has apparently been found.